The invention relates to a method of reducing interference in a radio system comprising at least one base station and a subscriber terminal including a receiver and a transmitter in connection with one another, in which transmitter the data to be transmitted is converted into symbols and each symbol is multiplied by a connection-specific spreading code composed of chips.
The invention also relates to a radio system comprising at least one base station and subscriber terminals comprising a receiver and a transmitter arranged to multiply the data to be sent by a connection-specific spreading code, which data comprises symbols and which spreading code comprises chips.
In CDMA (Code Division Multiple Access), a narrow-band signal of a user is multiplied to a significantly broader band by a spreading code. The users transmit simultaneously in the same frequency band. On each connection between the base station and a subscriber terminal, a different spreading code is used, and the signals of the users can be separated from one another in the receivers on the basis of the spreading code of each user. The spreading codes are chosen in such a way as to correlate with one another as little as possible. The signals multiplied by another spreading code do not correlate in an ideal case and do not restore to the narrow bandwidth, but appear as noise.
In a conventional CDMA, however, cross-correlations between different spreading codes are typically too high, since duration in time and usually also the length in chips of the mutually corresponding spreading codes is the same in different connections. This is a problem especially during a synchronizing phase which becomes more difficult because of the mutual interference between the users
The object of the present invention is to implement a method and a radio system wherein a cross-correlation between spreading codes is reduced during a synchronizing phase, whereby synchronization becomes easier and it is also possible to reduce interference.
This object is achieved by the method introduced in the preamble characterized by multiplying the data to be transmitted of at least one connection in the synchronizing phase by the spreading code whose duration in time differs from the corresponding spreading code in other connections by some chips at the most in such a way that the cross-correlation of the spreading codes receives different values at different periods of time.
A radio system of the invention is characterized in that it is arranged to employ a spreading code in at least one connection in the synchronizing phase, the duration in time of the spreading code differing by some chips at the most from the corresponding spreading code in other connections in such a way that the cross-correlation of the spreading codes receives different values at different periods of time.
The method of the invention provides many advantages in a radio system. The cross-correlation of the spreading codes can be substantially reduced in the synchronizing phase, whereby synchronization is improved and interference is also reduced.